Field of the Invention
This disclosure relates to an exhaust gas turbocharger for an internal combustion engine. More particularly, this disclosure relates to a heat shield for a variable turbine geometry turbocharger having a mixed flow turbine wheel.
Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting the horsepower of the engine without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of horsepower as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of a cleaner environment.
Turbochargers typically include a turbine housing connected to the exhaust manifold of the engine, a compressor housing connected to the intake manifold of the engine, and a center bearing housing disposed between and coupling the turbine and compressor housings together. A turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft is radially supported for rotation in the center bearing housing, and connects the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines a line which is the axis of rotation. As the compressor impeller rotates, it increases the air mass flow rate, airflow density and air pressure delivered to the cylinders of the engine via the engine intake manifold.
At low speeds there is often a delay before turbocharger boost is actually provided to the engine. This is called turbolag. To address turbolag, turbochargers may include variable turbine geometry. A variable turbine geometry turbocharger has movable guide vanes located outboard (e.g., upstream) of the turbine wheel. When the guide vanes are in the closed position, the air is directed toward the turbine wheel through narrow openings. This increases the air speed causing the turbine to spin faster than it would if the vanes were open. When the guide vanes are in the fully open position, the turbine is allowed to reach a maximum flow volume. Adjustment of the vanes helps to eliminate turbo lag and allows the turbine to provide reasonable pressure boost at both low and high engine speeds.
In a turbocharger, the turbine wheel may be a radial flow wheel in which the exhaust gas is directed along the radius of the turbine wheel, an axial flow wheel in which the exhaust gas is directed along the axis of the turbine wheel, or a mixed flow wheel in which some exhaust gas is directed along the radius of the turbine wheel, and some exhaust gas is directed along the axis of the turbine wheel. Frequently, the bearing housing is shielded from the heat of the exhaust gases by a heat shield which is placed between the turbine wheel and the bearing housing. Additionally, the heat shield can also function to direct exhaust gas toward the turbine wheel. However, due to the position of the heat shield with respect to the gas flow path through the turbocharger, the heat shield shape can affect turbine efficiency.